The overall purpose of this project is to help elucidate the relationship between structure and function of viral and ultimately cellular nucleoprotein complexes. The interaction between cellular expression and viral genome structure will also be analyzed, in both bacterial and animal virus systems. In addition to biochemical approaches, I plan to take advantage of the permeability of cells and virus particles to psoralen derivatives such as 4,5',8-trimethylpsoralen (trioxsalen). These components cross-link the two strands of DNA in the presence of long wavelength ultraviolet light and have been shown to be useful probes for the intracellular "nucleosome" structure of cellular chromatin and viral DNA-protein complexes. They also provide a powerful tool for and analysis of the mechanisms of cellular recombination and repair. Because of both the specificity of psoralen compounds for accessible regions of DNA such as internucleosomal bridges, and the ability to study intracellular and viral structure in vivo without prior disruption of the biological system, this approach will allow a detailed evaluation of the significance of the results from in vitro studies of nucleosome structure. The projects to be undertaken can be divided according to the approach and system employed: 1. The structure of virion and intracellular SV40 nucleoprotein complexes will be analyzed in order to study the relationship between nucleoprotein structure and the regulation of transcription and replication systems. 2. The intracellular structure of procaryotic "chromatin" will be investigated with respect to its protection from photoaddition of trioxsalen derivatives. A comparison will be made between E. coli DNA, lambda prophage DNA and superinfecting lambda DNA. 3. Photochemical inactivation of several virus classes will be used as a probe for the relationship between structure and biological activity. These studies will include examples of DNA and RNA phage, and DNA and RNA tumor viruses.